The Volt is an electric-drive vehicle, powered by a 16 kWh Li-ion battery, incorporating an internal combustion engine and generator to serve as a range extender; an electrical plug is intended to be the primary source of stored energy used to deliver motive power. (The Volt could also be called a plug-in series hybrid, depending upon your taxonomic preference.)

Given that GM had committed to a combustion engine and a generator as a range extender for the battery, the engineering team set out to develop a drive system that could maximize the combined efficiency of all the components under the different driving modes (all battery electric, and extended range). Put another way, the GM team wanted to extend the range of the vehicle as efficiently as possible, while maintaining quality driving dynamics and experience.

The story of the electric drive is really a story about efficiency. How do we take all this battery energy...and very efficiently and effectively drive the wheels. That’s ultimately what the customer is looking for, is to maximize this notion of electric range, to maximize this notion of efficiency of the generator when we have to use it.

—Pam Fletcher, Global Chief Engineer for Volt and Plug-In Hybrid Electric Powertrains

Mechanical losses such as friction and windage dictate that the efficiency of an electric motor declines somewhat as motor speed increases. Recognizing the benefits derived from their two-mode approach in their earlier hybrids, the engineering team also observed that they had a second motor—the generator motor—“going along for the ride”, as Fletcher said during her presentation at the launch event in Detroit. They thus decided to exploit the generator motor more thoroughly than it would otherwise have been if used exclusively as a generator in extended-range driving.

The resulting Volt drive unit consists of two motors—a 111 kW main traction and 63 kW (at 4800 rpm) generator motor (55 kW generator output)—as well as three clutches and a planetary gear set tucked in the end of the traction motor that improve overall efficiency by reducing the combined rotational speed of the electric motors as needed.

This engine obviously has the capability of revving much higher and producing much more output. But this is really a study in rightsizing, rightsizing an internal combustion engine for this extended range capability. We’re almost repurposing an internal combustion engine to provide this very unique type of propulsion. That’s how much power output we determined we needed for this car that has a very large battery and almost a half-size engine in terms of displacement to provide you with the average power required to provide an urban and highway type commute.

—Pam Fletcher

This configuration reduces battery drain at steady state, cruising speeds in a window ranging from around 30 mph to more than 70 mph (48 to 113 km/h), adding up to two miles (3.2 km) of additional all-electric range. The Volt delivers a pure-electric range between 25 and 50 miles (40 and 80 km)—depending on terrain, driving techniques, driver comfort requirements (e.g., HVAC), and weather. The range extender pushes that to approximately 350 miles (563 km).

The Volt’s drive unit uses an on-axis configuration; motors and gear-set are mounted in an in-line with the range-extending internal combustion engine. Two of the clutches are used to either lock the ring gear of the planetary gear-set or connect it to the generator/motor depending on the mode. The third clutch connects the internal combustion engine to the generator/motor to provide range extension capability.

The 111 kW traction motor is permanently connected to the sun gear, and the final drive (gear reduction, differential) is permanently connected to the planetary carriers. The planetary carrier gears are used to modulate gearing ratios between the vehicle’s electric motors, its internal-combustion engine, and its 2:16 final drive.

The Volt has two primary driving modes:

• All battery-electric (charge depleting), in which the battery is the sole source of power for the motors; and
• Extended-range (charge sustaining), in which the battery and engine work together in different operating modes to power the traction motor and to improve overall efficiency.

Each of these two driving modes is supported by two drive unit operating modes: a low-speed, 1-motor mode, and a high-speed, 2-motor mode.

Mode 1. Click to enlarge.

Mode 1: Low-speed EV Propulsion (Engine Off). In this mode, the ring gear is held (locked) by clutch C1. With clutch C2 and C3 disengaged, the generator-motor is decoupled from the engine as well as the planetary gearset. As the traction motor is permanently coupled to the sun gear, the planetary carriers must rotate when the traction motor rotates. Since the planetary carriers are permanently coupled to the final drive, the traction motor propels the vehicle. The generator-motor and the engine are idle during this mode, although the engine is free to start if necessary (example: engine maintenance mode).

Virtually all of the vehicle’s motive power is therefore delivered by the traction motor in this mode, including hard accelerations, using power supplied by the battery pack. With this configuration, the traction motor can produce up to 111 kW (149 hp) and deliver up to 370 N·m (273 ft-lb) of torque.

Mode 2. Click to enlarge.

Mode 2: High-Speed EV Propulsion (Engine Off). As vehicle speed increases, motor speed and losses also increase. To engage both motors and preserve motor efficiency, clutch C1 is disengaged, allowing the ring gear to rotate. At the same time, clutch C2 is engaged, connecting the ring gear to the generator-motor. The generator-motor is then fed current from the inverter, and runs as a motor. The engine remains disengaged from the generator-motor.

This mode allows the two electric machines to operate in tandem at a lower speed than if the traction motor alone was providing torque. The speed of the traction motor in this mode drops to about 3250 rpm from 6500 rpm in the 1 motor mode, according to Fletcher.

This strategy allows the Volt to wring out as much as two extra miles of all-electric operation out of its battery pack, depending on operating conditions. However, switching from low-speed to high-speed EV mode requires the simultaneous operation of two clutches. GM’s experience with simultaneous clutch operation in their two-mode transmissions and transaxles was key to the development of the Volt’s transaxle control strategy.

Mode 3. Click to enlarge.

Mode 3: Low-speed Extended-Range Propulsion (Engine Running). Once the Volt’s battery pack has reached its minimum state of charge (SOC) (which varies depending on operating conditions), clutch C1 engages, locking the ring gear, and clutch C2 disengages, decoupling the generator-motor from the ring gear. At the same time, clutch C3 engages to couple the Volt’s 1.4 liter Ecotec range-extending engine to the generator-motor, so that it may be operated in generator mode.

During low speeds as well as hard accelerations, the traction motor propels the vehicle. The engine drives the generator-motor, and power to drive the traction motor is delivered by the generator-motor as well as the battery pack via the Volt’s inverter. Under most conditions, the generator will provide enough power to maintain minimum battery SOC, and therefore allow the vehicle to remain in this mode until it is plugged in.

Mode 4. Click to enlarge.

Mode 4. High-Speed Extended-Range Propulsion (Engine Running). The blended two-motor electric propulsion strategy used at higher speeds in EV driving has also been adapted for extended-range driving. In this mode, the clutches that connect the generator/motor to both the engine and the ring gear are engaged, combining the engine and both motors to drive the Volt via the planetary gear set. All of the propulsion energy is seamlessly blended by the planetary gear set and sent to the final drive.

This novel mode—which GM calls “combined mode”—enables a 10-15% improvement in efficiency at steady state cruising speeds compared to a comparable single-motor mode, GM says. Under no circumstance can the Volt be propelled by engine torque alone; the traction motor must be operating if the vehicle is to move and the engine is to provide torque.

When we’re in this combination...on this planetary gearset we are driving the engine-generator combination onto the ring gear. We are utilizing the traction motor to provide the reactionary force so that we can ultimately drive the output. That is what happens in combined mode, that’s what allows us to get the 10-15% more efficiency.

—Pam Fletcher

In this mode, the generator still continues to produce electricity as well as deliver torque via the gearset, Fletcher said. The ratio of torque to power generation varies with operating conditions, and is, as the rest of the system, under the management of the control software, according to her. As noted earlier, the control software and architecture enabling this drive unit is critical to its overall success. We anticipate that additional information will be disclosed about the control mechanisms for Mode 4 as patents are awarded and SAE papers are approved for publication.

Packaging of the drive unit. The drive unit is quite compact, and includes the power electronics as well as the engine, motor generator, planetary gearset and traction motor. The power electronics unit includes three IGBT inverters: one for each motor, and one for the electric oil pump.

Driver experience. During the launch event (which GCC attended courtesy of GM), journalists paired off to drive pre-customer versions of the Volt on roads under different conditions for almost 200 miles. Based on that limited sampling, we can report that the transitions between modes are seamless and smooth; at one point, we had entered into range-extending mode without even knowing. The Volt accelerates crisply (and quietly), and handles snappily at moderately excessive interstate speeds—all on battery power.

The sole exception to the noise quality was on entering into mountain mode (driver-selected via the console); the engine races loudly.

The Volt offers three driver-selected modes: normal, sport, and mountain; mountain mode is designed to help the Volt traverse particularly steep and long grades—e.g., the Eisenhower Pass. This mode increases minimum battery SOC to around 45%. The driver will hear more engine noise during mountain mode, due to the higher rate of power generation required to maintain this mode. GM expects mountain mode to be required only under unusual power demand conditions.

GM engineers said that in the customer models, they are implementing a software fix to reduce the mountain mode noise somewhat. That said, GM wants the use of mountain mode to be exceptional—i.e., it doesn’t want customers running on mountain mode to recharge the pack. Power should come from the plug.